Big, popular forums start out as small forums. One day we will find that one
shard in our shared index is doing a lot more work than the other shards,
because it holds the documents for a forum that has become very popular. That
forum now needs its own index.

The index aliases that we’re using to fake an index per user give us a clean
migration path for the big forum.

The first step is to create a new index dedicated to the forum, and with the
appropriate number of shards to allow for expected growth:

PUT /baking_v1
{
"settings": {
"number_of_shards": 3
}
}

The next step is to migrate the data from the shared index into the dedicated
index, which can be done using a scroll query and the
bulk API. As soon as the migration is finished, the index alias
can be updated to point to the new index: